1. Field of the Invention
The present invention relates to a backlight unit that can reduce light losses that may be caused by leakage of the light to an external side. The present invention further relates to a liquid crystal display (LCD) with such a backlight unit.
2. Description of the Related Art
In recent years, an LCD has increasingly gained attention as a flat display device that varies optical anisotropy by applying an electric field to liquid crystal having a fluidity property as well as a crystal optical property. Since the LCD is relatively slim, with low electric consumption and capability of being made in a large-sized display panel with a high definition, it has been widely used.
Generally, such an LCD has a liquid crystal panel that displays a desired image on a screen by adjusting an amount of light fed from a backlight unit. The liquid crystal panel includes a plurality of liquid crystal cells arranged in a matrix pattern and a plurality of control switches for converting video signals that are to be fed to the liquid crystal cells.
In order to reduce a thickness of the LCD, the backlight unit used for the LCD has been made slimmer and light-weight. To accomplish this, a light emitting diode (LED) instead of CCFL has been used for the backlight unit since the light emitting diode is favorable in an electric power consumption, weight and brightness.
FIG. 1 is a sectional view of a related art liquid crystal display with an LED backlight unit and FIG. 2 is a view illustrating a light emission from a related art side emitter type LED.
As shown in FIG. 1, a related art liquid crystal display 1 includes a liquid crystal panel 19 having upper and lower substrates between which liquid crystal disposed and a backlight unit feeding light to the liquid crystal panel 19.
The backlight unit includes a plurality of LEDs 14, a metal core printed circuit board (MCPCB) 12 disposed under the plurality of the LEDs 14, a scattering plate 15 formed of polymethylmethacrylate (PMMA) to scatter the light generated from the LEDs 14, and a diffusing plate 16 for diffusing the light that has passed through the scattering plate 15. A variety of optical sheets 17 are disposed on the diffusing plate 16. A reflecting plate 13 is disposed between the LEDs 14 and the MCPCB 12 for reflecting light, which cannot transmit the diffusing plate 16 but is directed toward the MCPCB 12, toward the scattering plate 15.
The LEDs 14 are point light sources that emit red, green and blue lights. The MCPCB 12 is provided with a circuit for controlling the LEDs 14, and has relatively high heat dissipation property as compared with a conventional PCB. The MCPCB 12 functions to support the LEDs 14 as well as to control the light emission of the LEDs 14 using the circuit formed thereon.
A scattering optical pattern (not shown) is formed on a rear surface of the scattering plate 15. As shown in FIG. 2, the side emitting type LED 14 is designed to emit the light in a side direction rather than a vertical direction. However, in order to prevent the bright dot with respect to the light emitted in the vertical direction, the scattering plate 15 is provided to scatter the light as much as possible, thereby improving the brightness uniformity.
The diffusing plate 16 is spaced apart from the LEDs 14 at a predetermined distance so as to allow the light from the scattering plate 15 to have a more uniform distribution.
The reflecting plate 13 reflects the light that is reflected from the scattering plate 15 and/or the diffusing plate 16 and directed thereto or the light directly from the LEDs 14, thereby reducing the light loss.
In the above-described LCD 1, the light generated from the LEDs 14 is consecutively advanced to the scattering plate 15 and the diffusing plate 16, thereby providing the light having a uniform brightness to the liquid crystal panel 19.
As described above, the scattering plate 15 is formed of the PMMA to uniformly scatter the light generated from the LEDs 14. The PMMA is not transparent.
However, since the scattering plate formed of the PMMA has properties of a high coefficient of linear expansion and a low heat deformation temperature, it may be easily deformed under a high temperature/humidity environment.
The deformation of the scattering plate deteriorates the reliability of the backlight unit and thus the LCD.
Furthermore, the scattering plate formed of the PMMA cannot completely scatter the light generated in a vertical direction of the LEDs, thereby causing the poor bright spot.